Energy and environment are two critical issues we face in the 21st century. On the one hand, our daily lives and productivities are becoming more dependent on energy. This dependence will conflict with limited world oil reserves and increasing scarcity of supply. Recent surges in the international oil prices reflect lack of confidence in the future energy supply. On the other hand, the excessive use of fossil fuels causes global environmental pollution and climate changes. This further threatens the environment for human survival. Therefore, the development of clean renewable energy is one of the key technologies that would have the most impact on future world economy.
Energy is lifeblood of a nation's economy. Therefore, it is very important to carry out research and development for new energy materials and technology, and to actively search for clean renewable alternative energies. Currently, there is global enthusiasm in the research and development of new energy materials and technologies, focusing on the use of solar energy, fuel cells, lithium-ion batteries and super-capacitors. Among these, lithium-ion battery material technology has matured, and a sizeable market for such materials is rapidly forming.
Lithium-ion batteries entered the market in 1990s as a new generation of environmentally friendly batteries. Due to their unique properties of high operating voltages, low self-discharge rates, small sizes, light weights, and no memory effects, lithium-ion batteries are widely used as ideal power sources in the miniaturization of electronic devices, such as cameras, mobile phones, laptop computers, and portable measurement instruments. Lithium-ion batteries are also the preferred choice for use as light-weight, high-capacity power sources in future electric vehicles. In this regard, the cathode material of a lithium-ion battery is a key factor that can limit the overall performance of the battery.
The first commercial lithium-ion battery anode materials are mostly carbon or graphite materials. However, after the first charge and discharge, carbon and graphite would form a passivation film in the surface carbon layer, resulting in a loss of capacity. In addition, the potential of the carbon electrode and the potential of lithium are very close. When the battery is overcharged, lithium metal may precipitate on the surface of the carbon electrode, forming surface dendrites that can cause safety concerns.
In contrast, as anode materials for lithium-ion batteries, spinel Li4Ti5O12 does not have such problems. Li4Ti5O12 maintains high degree of structural stability during lithium ion intercalation and de-intercalation. It maintains the same spinel structure before and after lithium-ion intercalation, and it shows little changes in the crystal lattice parameters. Thus, the volume change is very small, and, therefore, Li4Ti5O12 is known as a “zero strain” electrode material.
As compared with carbon anode materials, which currently hold the largest market share, Li4Ti5O12 has a higher equilibrium potential, which can avoid the deposition of lithium metal, thereby avoiding dendrite formation and the hazards associated with dendrite formation. In addition, its platform capacity is more than 85% of the total capacity. Towards the end of a charging cycle, the potential rises rapidly, which can be readily used to indicate charge completion to avoid over-charging. Therefore, it has less safety concerns than the carbon anode materials.
Li4Ti5O12 has higher currents in charge and discharge performance and can be used as anode materials, together with LiCoO2, LiNiO2 or LiMn2O4, and activated carbon, etc., to form lithium ion batteries, all solid-state batteries, and hybrid super capacitors. It exhibits good application properties. At the same time, Li4Ti5O12 also has the advantageous properties of being resistant to over-charging, good thermal stability, and good safety performance. It has a wide range of applications in the field of electric vehicles, hybrid vehicles, and energy storage batteries.
However, Li4Ti5O12 itself has poor conductivity and does not have good performance in high-rate charge-discharge specific capacity. This greatly limits the applications of this material in lithium-ion batteries. Therefore, it is necessary to be modified this material to improve its electrical conductivity in order to improve its high-rate performance. At the same time, it is also necessary to maintain its high reversible capacity and good cycle stability, and to achieve the goal of cost.
Many methods are available for the synthesis of Li4Ti5O12. Common methods use solid-state reactions or the sol-gel method. The solid-state process is simple and easy to scale up. However, the drawback with the solid-state method is that the particle sizes of the products are difficult to control, and most are micron-sized particles with poor homogeneity. These are not conducive to high-current charge and discharge, and the high rate performance of such materials is poor. The sol-gel method requires addition of organic compounds, which increase the costs, and the process is complex. It would be difficult to achieve large-scale industrial production to meet the demand in the energy field with the sol-gel method.
Supercritical continuous hydrothermal synthesis (SHS) is a promising oxide preparation method. The products are of good quality and have high degree of crystallinity, and the particles are nanoscale particles. This method has the advantages of being environmentally friendly, fast and sustainable, etc. The method has been used to synthesize nano-metal oxides, including CeO2, AlOOH, Fe2O3, TiO2, CuO, and ZnO, as well as a number of complex metal oxides. The solubilities of the starting reactants for such syntheses are typically low. In supercritical water, the reaction materials disperse better, resulting in higher reaction rates. As a result, the products are of good qualities and are smaller particles in the nanoscale. The SHS method has been used in the preparation of cathode materials for lithium batteries. However, the use of supercritical continuous hydrothermal synthesis method to prepare lithium titanate anode material has not been reported.